The present invention is related to an injection molding machine, and in particular to the traversing cylinder for such a machine, which is capable of developing dual tonnage during mold separation.
In one well-known type of injection molding machine, a pair of die sections are connected, respectively, to a stationary platen and a moveable platen, the latter being supported for rectilinear movement on a plurality of strain rods, preferably four in number, and being rigidly connected to the stationary platen. The moveable platen is advanced and retracted with respect to the stationary platen by means of a traversing cylinder, which is supported on the backplate, the latter being rigidly connected to the strain rods and extending generally parallel to the moveable platen and stationary platen. The traversing cylinder comprises a cylinder barrel having a piston or plunger slidably received therein for rectilinear movement under the action of pressurized hydraulic fluid admitted into the cylinder barrel on opposite sides of the piston head. The forward end of the piston is connected to the moveable platen so that as the piston is advanced and retracted in the cylinder barrel, the moveable platen and the mold section carried thereby are moved toward and away from the stationary platen and its mold section.
In conventional prior art machines, the final clamping tonnage is achieved by a plurality of main cylinders associated with the stationary platen, which act through the strain rods, backplate, lockplate and compression columns to pull the moveable platen forwardly such that the mold sections are tightly clamped together with maximum holding force. After molding, a considerably large force is necessary to break the mold halves apart, and this is accomplished by the main cylinders, which retract the moveable platen slightly.
Since the piston size of the main cylinders is determined by the clamp tonnage for the machine, this is essentially the amount of tonnage which is available for developing the breakaway force needed to separate the mold halves. Because of machine design, this high breakaway force is only available for a very short distance, typically one-half inch, with some machines capable of up to two inches of breakaway force.
In certain molding operations, however, because of the particular product which is being molded, breakaway force over greater distances than this is needed. Although the standard traversing cylinder could be overbuilt so that high breakaway force could be exerted, this force is not necessary over the full reverse travel of the piston because during much of its travel, all that is necessary is to separate the mold sections sufficiently to enable removal of the molded article. Furthermore, high forces during retraction of the traversing piston require a much longer cycle time because a much greater volume of hydraulic fluid must be pumped into the cylinder barrel. Obviously, this is inefficient in terms of machine usuage and energy consumption.